A related apparatus is set out in U.S. Pat. No. 4,122,387. The disclosure set forth hereinbelow is an improvement over the mentioned reference. This apparatus utilizes digital circuitry to operate in a markedly different fashion to implement a shallow and deep resistivity measurement system. As set forth in the reference, a knowledge of the resistivity of the formation surrounding a borehole has great value. The resistivity is one of the key electrical properties which may vary laterally from the borehole, such variations in the lateral dimension being very useful in determining the nature and physical characteristics of the formation. As set forth in the reference, a deep measurement (that is relatively remote laterally from the borehole) yields apparent formation resistivity of the nearly uninvaded character. A different measurement is obtained from the shallow lateral depths, that is, those much closer to the borehole. This yields the resistivity of the invaded zone of the formation. Extremely shallow measurements of resistivity in the formation yield flushed zone resistivity. As will be understood, the three resistivities (in absolute and comparative terms) yield useful information for the evaluation of the formation.
As pointed out in the reference, variations in resistivity may indicate hydrocarbon migration in the formation in near vicinity of the borehole. Other valuable information can also be obtained from resistivity measurements. It is highly desirable therefore to have a common resistivity measurement apparatus on a sonde to be lowered in a borehole which measures laterally at relatively shallow and relatively deep distances. Guard electrode systems are positioned around current emitting electrodes on the sonde to focus the current flow through the conductive borehole fluid and thereby force the current flow to a desired depth in the formation.
Shallow penetration is obtained from a current emitting electrode with relatively short guard electrodes adjacent to it. The guard electrodes are arranged as a flanking pair. A similar but separate system is also included to direct the current flow much more deeply into the formation. This is accomplished by deploying a second pair of longer electrodes. The distance the current flows into the earth formation is a function of the length of the guard electrodes. A relatively long pair of guard electrodes provides greater lateral penetration. A shorter pair of electrodes has less lateral penetration. In fact, the penetration is, within limits, related to the size and spacing of the electrodes of the measuring system.
Resistivity is indicated by Ohm's Law. It is difficult to completely separate the currents which occur with two simultaneously operated current emitting electrode systems. Physical displacement of the two reduces electrode array interaction to some degree when the two are operated simultaneously. This interaction makes it difficult to achieve accurate shallow and deep measurements. For each measurement, it is necessary to determine the particular current flow from current emitting electrodes and voltage difference of the electrode with respect to a reference electrode. The four measurements required to obtain deep and shallow resistivity determinations cannot always be accurately obtained by physical displacement of the measuring systems along a common tool. However, in spite of the interaction, it is highly advantageous to operate two measuring systems on a common sonde because this reduces drilling interruption. Thus, with the device mentioned in the reference, the simultaneous operation of the deep and shallow measuring systems is achieved with some loss of accuracy.
One feature of the present apparatus is that the two resistivity measurements are made in separate discrete time intervals. Even so, there is no gap or omission of data because deep and shallow measurements are made at a rate that is sufficiently fast to accurately depict the subsurface earth formation features.